1. FIELD OF THE INVENTION
The present invention relates to a multistage fluid machine, and, more particularly, to a multi-stage fluid machine with a reduced noise the level.
2. DESCRIPTION OF THE PRIOR ART
Multistage fluid machines include a single-shaft multistage centrifugal compressor, with the compressor of this type has a casing, a rotary shaft supported by the casing, a requisite number of impellers mounted on the rotary shaft, and a driving device for rotating the rotary shaft. In this compressor, the fluid which is sucked from its suction nozzle is pressurized by a first impeller and is then sucked by a second impeller for a subsequent stage where it is further pressurized. The same operation is repeated until the fluid is finally discharged from a discharge nozzle. Thus, the compressor converts the energy imparted by the driving device to an increase in the pressure of the fluid. However, part of the energy escapes from the compressor as noise. The generation of this noise will be described below.
Each of the impellers has a finite number of blades. Therefore, at the outlet of one impeller, the velocity of the fluid is distributed in the circumferential direction in accordance with the number of blades of the impeller, and this periodic fluid velocity distribution rotates together with the impeller. Consequently, a stationary wall receives from the fluid an exciting force having a frequency expressed by a product of the number of blades of one impeller and the rotational speed thereof (this frequency being called a blade passing frequency and referred to hereinafter as BPF). The exciting force having the above-described frequency is transmitted to a casing through diffuser vanes and diaphragms, vibrating the surface of the casing. The vibrations generated on the surface of the casing are noises.
Another type of noise is caused by the vortex flow sound of the fluid which is transmitted through the casing. However, the vibrations generated on the surface of the casing are the main cause of noise generation.
In addition to these noises, noise may also be caused by the fact that harmonic vibrations of low frequency occur in which the casing is vibrated as one lump Japanese patent Laid-Open No. 60-50299 discloses a technique for reducing the level of noise caused by these harmonic vibrations by adopting an arrangement in which the diffuser vanes or the blades of the impellers are out of correspondence with one another in sequence in the circumfernetial direction.
In a multistage fluid machine of the single shaft multistage centrifugal compressor type, since the casing is large, the level of noise caused by the oscillatory waves generated on the surface of the casing, i.e., the level of noise caused by the exicting forces having a BPF, is high. Further, since a fluid machine of the above-described type is rotated at a relatively high speed while the number of blades in one impeller is large, the BPF often reaches about 1 to 2 kHz which ensures that the resulting noise will be at a level which is most uncomfortable for any human beings in the vicinity. Accordingly, there has been a demand for a reduction in the level of noise associated with such a BPF. However, in the known multistage fluid machine, sufficient consideration has not been given to reducting the level of noise caused by vibrations generated on the surface of the casing due to the exciting forces associated with a BPF.